Most Lensed Quasars at $z>6$ are Missed by Current Surveys

Abstract

The discovery of the first strongly lensed $(\mu \approx 50)$ quasar at $z>6$ (J0439+1634) represents a breakthrough in our understanding of the early Universe. We derive the theoretical consequences of the new discovery. We predict that the observed population of $z > 6$ quasars should contain many sources with magnifications $\mu \lesssim 10$ and with image separations below the resolution threshold. Additionally, current selection criteria could have missed a substantial population of lensed $z > 6$ quasars, due to the contamination of the drop-out photometric bands by lens galaxies. We argue that this predicted population of lensed $z>6$ quasars would be misclassified and mixed up with low-$z$ galaxies. We quantify the fraction of undetected quasars as a function of the slope of the bright end of the quasar luminosity function, $\beta$. For $\beta \lesssim 3.6$, we predict that the undetected lensed quasars could reach half of the population, whereas for $\beta \gtrsim 4.5$ the vast majority of the $z > 6$ quasar population is lensed and still undetected. This would significantly affect the $z > 6$ quasar luminosity function and inferred black hole mass distributions, with profound implications for the ultraviolet, X-ray, and infrared cosmic backgrounds and the growth of early quasars.